Single crystal material, which is the starting material for fabricating many electronic components such as semiconductor devices and solar cells, is commonly prepared using the Czochralski (“Cz”) method. Briefly, the Czochralski method involves melting polycrystalline source material such as polycrystalline silicon (“polysilicon”) in a crucible to form a silicon melt and then pulling a single-crystal ingot from the melt.
During the process, precise amounts of dopants are added to the molten source material to modify the base resistivity of the resulting monocrystalline structure. Dopants are typically added to the molten source material in solid form, at least for p-type and n-type silicon. However, the use of solid dopants presents several drawbacks.
One drawback is a thermal shock resulting from the temperature difference between solid dopants and the molten source material. This thermal shock causes the molten source material to solidify underneath the solid dopant granules, creating “floating boats”. Additionally, quartz particles can form during the formation of the floating boats. These quartz particles may remain in the molten source material long after the floating boats have melted, resulting in crystal defects, such as dislocations, in the final monocrystalline structure. A further drawback resulting from the addition of solid dopants to the molten source material is contamination of the monocrystalline growing assembly. The impact of solid dopants on the surface of the molten source material causes the molten source material to splash out of the crucible and onto various components of the monocrystalline growing assembly, which may result in crystal defects or damage to components in the assembly.
Yet another drawback to using solid dopants is that many have relatively high evaporation rates, such as indium. Placing these dopants directly into the crucible with the semiconductor or other solar-grade material prior to melting causes the dopant to evaporate during the heating of the semiconductor or solar-grade material. Additional dopant must be added to compensate for the lost dopant, often in significant quantities, resulting in an inefficient use of the dopant. Additionally, the evaporated dopant condenses on various components of the growing assembly, resulting in contamination of the assembly.
In view of the foregoing, it can be seen that a need exists for a simple, cost effective approach for introducing dopants into a melt of semiconductor or solar-grade material.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.